1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an active matrix (AM) device and an AM device containing the composition. The composition has a nematic phase and a positive dielectric anisotropy.
2. Related Art
In a liquid crystal display device, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source includes a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes a relationship between the general characteristics of the two. The general characteristics of the composition will be explained further based on a commercially available AM device. A temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or more and a desirable minimum temperature is approximately −10° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of aLiquid Crystal Composition and an AM DeviceGeneral CharacteristicsGeneral CharacteristicsNoof a Compositionof an AM Device1Temperature range of a nematicUsable temperature range is widephase is wide2Viscosity is small1Response time is short3Optical anisotropy is suitableContrast ratio is large4Dielectric anisotropy is largeThreshold voltage is low, electricpower consumption is small,and a contrast ratio is large5Specific resistance is largeVoltage holding ratio is large anda contrast ratio is large6It is stable to ultraviolet lightService life is longand heatNotes:1A liquid crystal composition can be injected into a cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the device. A product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to maximize the contrast ratio. A suitable value of the product depends on the kind of operation mode. In a device having a TN mode and so forth, a suitable value is approximately 0.45 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio. In general, a larger dielectric anisotropy provides a larger viscosity. A suitable dielectric anisotropy is demanded depending on the purpose of the device. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it has been used for a long time. A stability of the composition to an ultraviolet light and heat relates to a service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life.
Conventional compositions are disclosed in the following patent documents: JP 2001-3053 A/2001 (EP 1046694 A1), WO 2005017067 A1, JP 2004-285353 A2/2004 (EP 1454975 A2) and WO 2005007775 A1.
A desirable AM device is characterized as having a usable temperature range that is wide, a response time that is short, a contrast ratio that is large, a threshold voltage that is low, a voltage holding ratio that is large, a service life that is long, and so forth. Even a one millisecond shorter response time is desirable. Thus, a composition having characteristics such as a high maximum temperature of a nematic phase, a low minimum temperature of a nematic phase, a small viscosity, a large optical anisotropy, a large dielectric anisotropy, a large specific resistance, a high stability to an ultraviolet light, a high stability to heat, and so forth is especially desirable.